1. Field of the Invention
The present invention pertains to methods and apparatus for forming a temporary fluid tight plug in a conduit such as a fluid transmission pipeline disposed in a submarine environment wherein a liquid slug or isolating pig is frozen within the line by the circulation of a cryogenic fluid through a housing which is temporarily placed on the line. Alternatively, the fluid circulation housing may be disposed in a habitat adapted for circulation of the cryogenic fluid and control of the freezing process.
2. Background Art
The present invention pertains to an improved method and apparatus for providing temporary plugs in closed generally cylindrical conduits and particularly in relatively large diameter fluid transmission lines for purposes of installation, repair, modification and hydrostatic testing thereof. The formation of a temporary substantially fluid tight seal or plug in a fluid transmission pipeline by freezing a slug of liquid within the interior of the pipe is a viable process. One successful method is disclosed in U.S. Pat. No. 3,827,282 issued Aug. 6, 1974 to B. D. Brister. Improved apparatus for carrying out various processes in forming a plug of frozen liquid in a section of a pipeline or the like is disclosed also in U.S. Pat. Nos. 4,112,706 and 4,220,012 also issued to B. D. Brister.
The aforementioned patents contemplate the formation of a frozen slug of liquid which may be provided using the liquid in the pipeline itself or by injecting a quantity of water or other freezable liquid into the line between, for example, spaced apart batch pigs. The slug of liquid is transported to the desired freezing location and a housing is placed around the exterior of the pipeline, which housing is adapted for the circulation of a cryogenic fluid to freeze the liquid within the line at the desired site.
The substantial number of fluid transmission lines submerged in various bodies of fresh and sea water have presented new problems in carrying out installation, repair, modification and testing of these lines. Moreover, the transportation of highly volatile fluids through pipelines in various environments also presents, in many instances, hazardous working conditions for personnel attempting to carry out the desired procedures. In both instances it has become highly desirable to provide for the remote control of the process of forming the fluid tight seal or plug.
The modification and repair of fluid transmission lines disposed in submarine environments has been a particular problem due to the inaccessibility of the line to the equipment needed for circulating the cryogenic fluid around the exterior of the line to thereby freeze the liquid slug within the line or, alternatively, a quantity of liquid disposed within a closed container of one type or another deployed within the line. Although certain techniques and equipment have been developed for providing subsea enclosures or habitats whereby workers may have access to a section of a fluid transmission line without working in diving gear, prior art techniques for forming a plug in a section of line within the enclosure or habitat have not been entirely successful and have been somewhat dangerous.
However, in accordance with the present invention improved methods and apparatus have been developed for carrying out various operations on fluid transmission lines located in submarine environments as well as in areas wherein it is desirable to have operating personnel located remote from the location of the plug or seal for as much of the operating time as possible. Some of the latter mentioned environments considered are any areas where highly volatile or otherwise dangerous fluids are being conducted through a pipeline including offshore petroleum producing platforms as well as fluid transmission lines located in hazardous environments such as areas where toxic substances are being produced or are likely to be vented, including nuclear processing or power plants.